1. Field of the Invention
This invention relates to an image information transmitting system and more particularly to a system for continuously transmitting a temporally correlated group of image planes.
2. Description of the Related Art
In transmitting information such as image information, it is always the theme of efforts to reproduce the original information with a higher degree of fidelity with a smaller amount of transmitting information. Hence, varied kinds of transmission methods have been proposed for this purpose.
These methods include adaptive type variable density sampling methods of appropriately changing sampling density, that is, varying the density of information being transmitted. An example of this method has been disclosed and known by the name of a time axis transforming band compressing method (hereinafter referred to as TAT method). The TAT method is briefly described below:
FIG. 1 of the accompanying drawings shows the fundamental concept of the TAT method. An original signal is divided as indicated by broken lines into blocks by a predetermined period of time. The information contained in the original signal within each divided block is checked to discriminate its degree of density. When any of the blocks is thus found to be dense, data obtained by sampling the original signal of the block is completely transmitted as transmitted data. For a block determined to be sparse, only a portion of data is transmitted while the rest is regarded as thinned-out data and is not transmitted.
The arrangement according to this concept decreases the amount of data to be transmitted per unit time and thus permits the transmitted signal to be band compressed. The data thus transmitted is used by the receiving side for forming data corresponding to the thinned-out data. In other words, some interpolation data which is in proximity to the thinned-out data is obtained by computation by using the transmitted data. Since the interpolation data corresponds to a sparse part of the information signal, it is in close proximity to the thinned-out data. Compared with a case where the whole data is transmitted, the interpolating arrangement gives a restored signal with a fairly high degree of fidelity to the original signal while the transmission band can be reduced to a great degree by the arrangement. In other words, the amount of information to be transmitted is reduced by the arrangement.
Meanwhile, the elaborateness or fineness of the original signal within each of the divided blocks are examined in making a discrimination between transmitting the whole sampling data and transmitting just a portion thereof. Information on the result of this discrimination is also transmitted along with the transmitted data as transmission mode information.
In the case of image information, transmission according to the above-stated concept is performed in the following manner: The image information has a two-dimensional spread and has a correlativity between horizontal and vertical directions. Therefore, transmission of image information can be more effectively accomplished by arranging the intervals of sampling to be variable not only in the horizontal direction but also in the vertical direction. This idea will be called the two-dimensional TAT method. The following is the brief description of the two-dimensional TAT method:
FIG. 2 is a data transmission pattern of the two-dimensional TAT method. In this method, one picture plane is divided into a plurality of picture element blocks. Each of the divided blocks consists of an m.times.n number of picture elements. The transmitted data density of one picture element block is arranged to be variable from another and independently of another. In the case of FIG. 2, each picture element block consists of 4.times.4 picture elements and is arranged to be transmissible in two different transmission modes. In FIG. 2, each mark ".circle." represents a picture element to be transmitted and another mark "X" a picture element to be thinned-out. A reference symbol E denotes a transmission pattern in which data of all the picture element is transmitted; and another symbol C a pattern in which only a portion of data of all the picture elements within one block is transmitted. Hereinafter, the mode of transmission in the former pattern will be called the E mode and transmission in the latter the C mode respectively. As apparent from the illustration, data is transmitted in the C mode with 1/4 of the information transmitting density of the E mode. In the case of the C mode, the original image plane is restored by forming interpolating picture element data for each of the thinned-out picture elements on the basis of the transmitted data representing a picture element located near to the thinned-out one within the same picture element block. A system for carrying out the two-dimensional TAT method is arranged as described below with reference to FIG. 3:
FIG. 3 is a block diagram showing by way of example an analog transmission system. An incoming image signal is sampled for all the picture elements thereof by an analog-to-digital (hereinafter referred to A/D) converter 1. By this, data for all the picture elements is generated. This all-picture-element data is supplied to a thinning-out circuit 2. The thinning-out circuit 2 performs a thinning-out operation in a manner corresponding to the C mode pattern shown in FIG. 2. The circuit 2 thus produces C mode picture element data. The C mode picture element data is supplied to an interpolation circuit 3, which performs a computing operation to obtain interpolation picture element data corresponding to the thinned-out picture elements. The interpolation picture element data is supplied to a mode discrimination circuit 4 together with the all-picture-element data produced from the A/D converter 1. Then, each picture element block is determined whether it is to be transmitted in the C mode or in the E mode. At the mode discrimination circuit 4, computation is performed for each of the picture element blocks to obtain a difference between the picture element data produced from the A/D converter 1 and the interpolation picture element data. The sum of the difference (hereinafter referred to as a block distortion) is computed for every picture element block and then a total difference thus obtained for one field portion of the signal is stored in a memory.
Before arrival of the data of a next field, the distribution of block distortions of all the picture element blocks is thus obtained. In this instance, the ratio of the number of picture element blocks to be transmitted in the C mode to that of the picture element blocks to be transmitted in the E mode must be arranged to be unvarying to fix the rate of compression. For example, assuming that 2/3 of all the picture element blocks are to be transmitted in the C mode and 7/8 of these blocks to be transmitted in the E mode, a total number of transmission data (or the rate of compression) becomes (2/3.times.1/4+7/8.times.1=)1/2. Therefore, in accordance with the distribution of the block distortion covering all the picture element blocks, a threshold value of distortion is predetermined for determining a boundary between the C mode and the E mode.
Following this, at the time of arrival of the incoming image signal for the next field, the stored block distortion values are read out one after another and compared with the threshold value to determine thereby the transmission mode to be selected. In case that the read out distortion value coincides with the threshold value, the transmission mode is determined in such a manner that the number of the picture element blocks to be transmitted in the C mode and that of the blocks to be transmitted in the E mode are in the predetermined ratio. The mode discrimination circuit 4 produces a mode discrimination signal representing the determined transmission mode.
The mode discrimination signal which is thus obtained in the above-stated manner is supplied to a switch 7. Then, the picture element data is selectively read out from a buffer 5 which is provided for the picture element data of the E mode and a buffer 6 which is for the picture element data of the C mode. The output of the switch 7 is supplied as the transmission data to a digital-to-analog (D/A) converter 8 to be converted back into an analog picture element signal. This signal is then produced to a transmission line. Further, the mode discrimination signal is also produced to the transmission line via a buffer 9 as a mode information signal.
FIG. 4 shows in outline the arrangement of the receiving side of the two-dimensional TAT transmission system. The picture element signal which has been processed in the manner as described in the foregoing and supplied via the transmission line is received at an A/D converter 10 to be converted back into a digital picture element data. The output of the A/D converter 10 is supplied to a C mode interpolation circuit 11. The circuit 11 performs a computing operation to obtain interpolation data corresponding to the thinned-out picture element data in the C mode.
Meanwhile, the transmitted mode discrimination signal or mode information signal controls a switch 12. The connecting position of the switch 12 is shifted to its one side E when the signal indicates the E mode and to the other side C thereof when the signal indicates the C mode. Through this switch 12, the whole picture element data including the E mode picture element data, the C mode picture element data and the interpolation picture element data is stored gradually at a frame memory 13. The stored data is read out from the frame memory 13 in a sequence, for example, according to a television signal. The read out data is produced via a D/A converter 14 to become an image signal.
As described above, the image information can be effectively transmitted by the transmission system operating according to the two-dimensional TAT method. However, when a television signal which is obtained in the manner described above is displayed, deterioration becomes conspicuous in resolution in a still picture region although the resolution is acceptable in a motional picture region. Meanwhile, in the still region on the image plane, there is a high correlativity in the time axial direction. A method of utilizing this correlativity in the time axial direction has recently been advanced.
However, in the transmission system of the above-stated two-dimensional TAT method, image planes having temporal correlation among them are arranged to be continuously transmitted even for a still picture part having a high degree of temporal correlation without making any distinction between a still part and a motional part of each image plane during the continuous transmission of a temporally correlated group of image planes. Therefore, in the event of a still picture part having an extremely high degree of correlativity among image planes on the time base, transmission of similar image information signals are unnecessarily repeated many times. This results in a very poor transmission efficiency.